1. Field of the Invention
The present invention relates to digital signal processing for a multiple-valued image signal.
2. Description of the Related Art
In image transmission including a recording and reproducing of image data, the image data is compression-encoded in order to reduce the amount of data. The image data that is compressed-encoded may have a dynamic range that is different.
For example, a digital transmission specification “REC656” is prescribed to use a range of 16-235 in a range of 0-255 using 8-bit image data. That is, in the digital transmission specification, a black level is set to 16 and a white level is set to 235. The 8-bit image data which is not based on this transmission specification has the dynamic range from 0 to 255. Also, an image input apparatus can set up the dynamic range using a manual. The image data which has various dynamic ranges as described above can exist together.
In cases where compression encoding is performed for the image data, without taking the dynamic range of the image data into consideration, image quality deterioration, such as a reduction of contrast and an artifact where should be true black, has occurred by the compression encoding.
The following image processing has been proposed as a method of solving the above-described problem. The density histogram by the pixel value of image data is calculated and the pixel values according to white and black are predicted by the histogram. A dynamic range is determined from the pixel values and the parameter of encoding is set up based on the dynamic range. Since each image does not always have both the white level and the black level in a screen by the above-described method, it is difficult to determine a suitable encoding parameter certainly and stably. The apparatus needs to calculate the density histogram and there is a problem that there are large loads on hardware (such as a memory and a computing element) and software.
Also, in order to correct the difference of the display properties between different display apparatuses, the encoding apparatus which can correct the difference of the dynamic range and gamma characteristics between display apparatuses has been proposed. In the above-described apparatus, the image data with which the dynamic ranges are different cannot be handled simultaneously.
Recently, use of photographing apparatuses that can select still image recording and moving image recording has spread. The dynamic range of image data is different in the still image recording and the moving image recording. If the still image and the moving image are concatenated and edited, the image quality deterioration, such as the reduction of contrast or over-contrast (it means that the image is too bright or too dark), is caused from the difference of the dynamic range. Thus, using the above-described apparatuses and methods, it is difficult to obtain a good quality edit of an image.